When customizing your Foswiki installation, you can either choose to completely rewrite all css styles, or to selectively override some styles. The latter option is by far less time consuming, but all depends on your graphical wishes.

Set the value of macro PATTERNSKIN_THEME_VARIANT to that topic attachment url using %PUBURL%/YourWebName/YourTopicName/your_theme_variant_file.css

Note: If using %WEBHEADERART% to customize the top bar, some #patternTopBarContents CSS is expanded inline from templates/css.pattern.tmpl. Override of some attributes, such as background color and repeat are not possible from the CSS files.